My first is a man on the moon. That's it, there's just a guy in a spacesuit walking around on the moon. No identifiable markings on the suit or anything. If you look at him through a telescope, about 1.3 seconds later, he'll turn and wave hi, but otherwise continue walking.

Second is a sealed entity sort of thing. Basically an unearthed tomb contained a huge locked box impaled with swords and painted with stylized chains. There's a small, immovable bowl in the middle of the floor where the box is and if you pour blood into it, it absorbs it and repaints the chains, which continuously, gradually fade. (I'll work out timeline issues later).

Third is a sapient/sentient liquid metal (I haven't decided how intelligent to make it yet). It's capable of levitation and that's how it gets around. It can move through metals a-la 106, but is contained by hard plastic. It attacks by extending hardened 'lances' of itself through warm blooded animals and absorbing something (haven't decided if it's blood, silicon, or some other substance yet) and transmuting that into more of itself. It prefers to stay together as one large mass, but it has shown able to split apart if it grows too large, or into minuscule particles in order to more effectively attack (like having humans breath it in, then converting inside the body and exploding outwards).

As for the last one, there are a metric buttload of useful minerals and elements in the human body. Just because a chemical is not a metal, does not mean it can't be useful to a theoretical metal-based creature. Thank you AP Chemistry.

As has been said, the first idea has been done so well it isn't worth trying something too similar. The second— hrm. We have quite a few lock box of horror SCPs, and the god-we-must-sacrifice-infants-to-or-kablooey makes the demands of a bowl of blood pale in comparison. You might be able to make that work if you had a really distinctive twist or an unusual discovery log, but the idea on its own is similar to a lot of other things.

I'm not too keen on the third idea— it's just kinda dull. The only thing that catches my attention is the respiratory detail— something that reproduces/takes shelter by being inhaled. That's very Mushishi and Black Death reminiscent.

If I had to choose one out of the three, I'd probably choose the third one. If you focused on that last bit, and applied some elbow grease, cowbell, and a willingness to gain carpal tunnel, you could end up with a very nice SCP. I'm imagining people either melting, or evaporating into this stuff.